1. Field of the Invention
The present invention generally relates to an image forming apparatus, and more particularly, to a transfer belt of an image forming apparatus for sensing an initial printing position without an error.
2. Description of the Related Art
As shown in FIG. 1, a typical image forming apparatus comprises an exposure unit 12 for generating laser beam, a photosensitive medium 21 on which electrostatic latent image is formed, a development unit 31 for supplying a developer to the photosensitive medium 21 to form visible image, a transfer unit 41 for transferring the visible image of the photosensitive medium 21 to printing paper P, and a fixing unit 51 for fixing the visible image to the printing paper P. The transfer unit 41 includes a transfer belt unit 42, a first transfer roller 43 and a second transfer roller 44. Additionally, the transfer belt unit 42 includes a transfer belt that serves as the transporting medium for the visible image and a plurality of rollers 46 supporting the transfer belt.
In the image forming apparatus having the configuration described above, an electrostatic latent image is formed on the surface of the photosensitive medium 21 when the photosensitive medium 21 charged with a certain potential and is irradiated with laser beam. Thereafter, a visible image is formed when developer supplied from the development unit 31 is applied to an image forming portion of the electrostatic latent image, the visible image is transported to the transfer belt, and transferred to the printing paper P. Finally, the visible image is fixed on the printing paper P by the fixing unit 51.
In such a printing operation, the transfer process is an important procedure which may affect the print quality, and the timing of forming the electrostatic latent image on the photosensitive medium should be synchronized with the transfer belt, because the electrostatic latent image on the photosensitive medium should be sequentially transported to the transfer belt without color unbalance before it is transferred to the printing paper P, thereby performing the printing.
Accordingly, by sensing a certain position of the intermediate transfer belt with a sensor and using this position as the criterion to determine when the electrostatic latent image should be formed on the photosensitive medium, the initial printing position of each color may be more accurately determined.
Conventionally, the initial printing position has been determined by sensing the tip of a position sensing hole in the transfer belt as shown in FIG. 2.
FIG. 2 is an elevation view of the intermediate transfer belt provided with a position sensing hole 11. FIG. 3 is a cross-sectional view of the transfer belt shown in FIG. 2, with the polycarbonate alloy 10 (hereinafter, to be referred to as PC alloy) of the intermediate transfer belt being formed with a position sensing hole 11 at one edge and tape 20 being attached to an upper side and a lower side at both edges of the PC alloy 10 so as to protect both edges of the PC alloy 10.
FIG. 4 is a cross-sectional view showing a position sensor 30 and a portion of the transfer belt having the position sensing hole 11. As shown in this figure, a portion of PC alloy 10 moves over a guide rail 40. The protective tape 20 is attached to the transfer belt on its upper and lower sides at both ends, although only one end is shown. When in motion, a portion of the transfer belt near an edge travels through the position sensor 30 and the position sensing hole 11 passes through the position sensor 30, the position sensor 30 senses the tip of the position sensing hole 11 and outputs the sensed signal. An image forming apparatus may use this outputted signal to determine the appropriate initial printing position for each color.
Since the position sensing hole 11 is formed by penetrating the PC alloy 10 (i.e., a throughhole), the segments of protective tape on the upper and lower sides of the transfer belt tend to stick to each other in the position sensing hole 11. However, due to the thickness (about 0.15 mm) of PC alloy 10, the tape segments do not stick together entirely. As a result, the position sensing hole 11 may be opaque. Additionally, when the segments of protective tape are continuously used, they tend to peel off at the attachment portion, and hence it is impossible to keep the section of the position sensing hole 11 transparent. Accordingly, the section of the position sensing hole 11 becomes to be opaque, thereby causing sensing errors in the sensing operations of the position sensor 30.
FIG. 5 shows another conventional transfer belt in which the PC alloy 10 is covered with the protective tape 20 on the upper and lower sides at both edges but in which the protective tape covering the hole 11 is removed so that the hole 11 is open. This arrangement does not suffer from the disadvantages of opacity described above. However, when the tape over the hole is removed as shown in FIG. 5, toner particles may enter and pass through the hole in the transfer belt and contaminate the drive roller (not shown). When an intermediate transfer belt unit is contaminated in this way, the belt slips and, as a result, is difficult to drive the transfer belt at the desired speed.